In the manufacture of semiconductor devices for use in the conversions of electric signal and electric power, silicon (hereinafter represented as “Si”) semiconductor materials are used in general, and various improvements are being made to those semiconductor devices so that they can be used even in more hostile temperature environments. Examples of such semiconductor devices include semiconductor devices installed in artificial satellites and so on and used in space, and high-speed highly-integrated microprocessors installed in computers. And further, other examples of those devices include various power integrated circuits which control electric power above hundreds of milliwatts and various individual semiconductor devices such as IGBT etc. installed in automobile engine rooms. Furthermore, in various light-emitting semiconductor devices which emit light with various wavelengths, compound semiconductor materials are used. In these light-emitting semiconductor devices as well, there is a tendency to heighten current densities in order to achieve higher-intensity light emission, and therefore improvements are being made so that they can be used in hostile temperature environments in which higher bonding temperatures are used.
On the other hand, attention is being given to wide gap semiconductor materials such as silicon carbide (hereinafter represented as SiC) because they have excellent physical characteristics such as the facts that SiC is larger in energy gap than Si and is also higher in dielectric breakdown field strength by an order of magnitude. Wide gap semiconductor materials are materials suitable for the manufacture of power semiconductor devices used in even more hostile temperature environments, and semiconductor devices made of these materials are also being actively developed in recent years.
As an example of conventional high-heat-resistive high-withstand voltage power semiconductor devices using SiC, a power semiconductor device having a SiC diode element described below is disclosed in “Proceedings of 2001 International Symposium on Power Semiconductor Devices & IC's, pp. 27 to 30” (Conventional Art 1). In such a SiC diode element, a pn junction, from which electric charges are injected onto a SiC substrate, is formed by using an epitaxial film produced by an epitaxial growth technique. After the epitaxial film in the end region of the substrate has been removed by means of mesa etching, a termination portion for use in electric field relief is formed by means of ion implantation. Specifically, a 0.7-μm-thick p-type epitaxial layer is removed in a mesa etching process using a depth of about 1 μm, and then a 0.4-μm-thick film made of an inorganic substance such as silicon dioxide is formed as a passivation film. In this conventional art, the SiC diode element with a high withstand voltage of 12 to 19 kV can be produced.
FIG. 5 is a cross-sectional view of a SiC diode device fabricated by housing the above conventional SiC diode element in a package. In FIG. 5, on the top surface of a metal support 93 having a cathode terminal 92 on its under surface, the SiC diode element 90 is attached through its cathode electrode 97. To the support 93, an anode terminal 91 is further provided so as to penetrate the support 93 in a state of being electrically insulated from the support 93 by an insulator 12. The anode terminal 91 is connected to the anode electrode 96 of the SiC diode element 90 via a lead wire 8. On the top surface of the support 93, a metal cap 94 is provided so as to cover the diode element 90, with which a space 95 in the package including the diode element 90 is sealed. The space 95 is filled with a sulfur hexafluoride gas. In the case where the filling is conducted by using the sulfur hexafluoride gas, a coverture 100, which is represented as a mountain-shaped substance of FIG. 5 and will be described later, is not provided.
The reason why the filling is conducted by using the sulfur hexafluoride gas will be set forth below. Since the creeping distance between the anode electrode 96 and the exposed sides 90a of the element 90 not covered with the passivation film 98 is short, discharging is readily brought about in air, and therefore its withstand voltage cannot be increased. In order to increase the withstand voltage, the package is filled with the sulfur hexafluoride gas which is least apt to cause discharging in a high electric field as an insulating gas. In cases where inert gases such as nitrogen gas and noble gases such as argon gas are used as insulating gases, these gases are lower than sulfur hexafluoride gas in maximum dielectric breakdown field, and thus discharging is brought about in the gasses at the time of the application of high voltages. As a result, the SiC diode element 90 itself and the passivation film 98 made of silicon dioxide or the like are damaged. Therefore, in order to increase the withstand voltage, the filling is conducted by using the sulfur hexafluoride gas extremely stable even at a high temperature of about 150° C. to prevent discharging and dielectric breakdown.    Patent Reference 1: Japanese Patent No. 3395456    Patent Reference 2: Japanese Patent No. 3409507    Non-Patent Reference 1: Proceedings of 2001 International Symposium on Power Semiconductor Devices & IC's, pp. 27 to 30